CHAPTER XIII 

 ENERGY AND CHEMICAL CHANGE 



IN the description of chemical changes, the fact that heat was 

 evolved has frequently been mentioned. In several instances a 

 current of electricity has been used to produce chemical change. 

 It is now necessary to collect these scattered facts and classify 

 them for future use. 



Physical Accompaniments of Chemical Change. When 

 iron and sulphur combined (p. 14), and when iron burned in oxy- 

 gen or copper in chlorine, much heat was developed. On the 

 other hand, the decomposition of mercuric oxide, as was pointed 

 out (p. 15), owed its continuance to the persistent application of 

 heat and ceased as soon as the source of heat was withdrawn. 

 Here, apparently, heat was consumed during the progress of the 

 change, and the chemical action was limited by the amount of 

 heat supplied. The production or consumption of heat may, 

 therefore, be a feature of chemical change. 



In the burning of iron or magnesium in oxygen, and in the ac- 

 tions of chlorine on copper and turpentine, light was also pro- 

 duced. Conversely, silver chloride can be kept any length of 

 time in the dark, but in sunlight it becomes first bluish and then 

 brown, simultaneously giving off chlorine gas and finally leaving 

 only silver as a fine powder. Silver bromide or iodide, in pho- 

 tographic plates, films, and paper, is changed by light in a similar 

 way, liberating the bromine or iodine. It would appear, there- 

 fore, that light may be given out or consumed in connection with 

 chemical change. 



We have seen (p. 55) that a current of electricity may be em- 

 ployed to decompose hydrochloric acid and other chlorides, and 



153 



